1. Field of the Invention
The present invention relates to a self-pulsation type semiconductor laser device. More specifically, the present invention relates to a self-pulsation type semiconductor laser device having a low level of noise, which is suitable as a light source for recording and reproducing operations of an optical disk or the like.
2. Description of the Related Art
When laser beams reflected from an optical disk or the like enter a semiconductor laser device which oscillates in a single longitudinal mode, an oscillation state is unstable and changes due to optical interference, resulting in the generation of noise. Such noise, referred to as "noise due to return light", significantly disrupts the performance of the semiconductor laser device in the case where it is used as a light source for the reproducing operation of an optical disk or the like.
In order to reduce the noise due to the return light, conventionally, a current oscillating at a high frequency (hereinafter, referred to as the "high frequency oscillating current") is overlapped with a driving current of the semiconductor laser device (referred to as the "high frequency overlapping method").
More specifically, when the high frequency oscillating current is overlapped with the driving current, a laser beam pulse train is generated in synchronization with the phase of the high frequency oscillating current. At this time, the laser oscillation mode is changed in an extremely short time (e.g., about 2 nanoseconds), and the wavelength of the laser beam pulse train is correspondingly changed in a discontinuous manner within a small range (e.g., within the range of .+-.2 nm from the central wavelength). As a result, at the time when the laser beams return to the semiconductor laser device after being reflected from an optical disk or the like (i.e., when the return light reaches the semiconductor laser device), coherence between the return light and the laser beams in an oscillation state disappears (i.e., the return light and the laser beams become incoherent), resulting in an unstable variation in the laser oscillation being suppressed. As a result, the generation of noise due to the return light can be prevented.
This method, however, requires a circuit dedicated for generating the high frequency oscillating current; therefore, this method is not suitable for realizing the miniaturization of a device to which the semiconductor laser device is to be incorporated (e.g., a reproduction apparatus for an optical disk).
In recent years, a semiconductor laser device not requiring a circuit for generating the high frequency oscillating current has been developed by utilizing the self-pulsation phenomenon of the semiconductor laser device. Such a semiconductor laser device is referred to as a self-pulsation type semiconductor laser device, and includes a saturable absorbing region in an optical waveguide portion of the semiconductor laser device. In the self-pulsation type semiconductor laser device, a laser oscillation mode periodically changes in an extremely short time by the function of the saturable absorbing region without overlapping the high frequency oscillating current with a dc driving current, thus achieving a low level of noise by a mechanism similar to the above-mentioned high frequency overlapping method.
Hereinafter, a conventional example of a self pulsation type semiconductor laser device will be described with reference to the accompanying drawings.
A semiconductor laser device shown in FIG. 8 includes an n-type GaAs substrate 41 and a semiconductor multilayered structure which is grown thereon.
This semiconductor multilayered structure includes an n-type buffer layer 42, an n-type first cladding layer 43, an active layer 44, and a p-type second cladding layer 45, on the substrate 41 in this order. The second cladding layer 45 includes a striped ridge portion, and the portions on both sides of the ridge portion (non-ridge portions) of the second cladding layer 45 are thinner than the ridge portion. A p-type contact layer 47 is formed on the ridge portion of the second cladding layer 45 via a p-type intermediate layer 46. An n-type GaAs buried layer 49 is formed on the both sides of the striped ridge. Lateral confinement of light in a horizontal direction is achieved by a difference in equivalent refractive indexes between the ridge portion and the non-ridge portion.
A p-type electrode 410 is provided on the upper face of the semiconductor multilayered structure, and an n-type electrode 411 is provided on the lower face of the substrate 41. A voltage is applied between the electrodes 410 and 411 so as to allow carriers necessary for generating laser beams to be injected into the active layer 44.
When a voltage is applied between the electrodes 410 and 411, reverse bias is applied to the pn junction between the p-type semiconductor layer and the n-type GaAs buried layer 49 in the semiconductor multilayered structure. For this reason, current does not flow through the GaAs buried layer 49 and is narrowed to the striped ridge portion. As a result, current flows through a selected region in the active layer 44 (i.e., a region located immediately below the striped ridge portion).
The region in the active layer 44 through which current flows to a level exceeding a predetermined level functions as "a gain region" for laser beams, and the other regions function as "a saturable absorbing region". The function of the saturable absorbing region will be described below.
The saturable absorbing region functions not as a gain region for laser beams, but as an absorbing region. At this time, an extent to which the saturable absorbing region absorbs laser beams (light absorption amount) depends on the density of photoexcited carriers which are present in the saturable absorbing region. Herein, the term "photoexcited carriers" refers to electrons and holes which are excited from a valence band to a conduction band by absorbing laser beams.
FIG. 9 shows the relationship between the light absorption quantity and the number (density) of photoexcited carriers. The light absorption quantity lowers as the photoexcited carrier density increases, and the light absorption quantity increases as the photoexcited carrier density lowers. When the light absorption quantity of the saturable absorbing region periodically changes, an internal loss of the semiconductor laser device also periodically changes. Thus, a threshold current density necessary for laser oscillation periodically changes. As a result, even if the driving current is kept constant, substantially the same effect as obtained in the case where the driving current varies can be provided, resulting in the achievement of self-pulsation.
However, the above-mentioned prior art poses the following problems.
In the structure shown in FIG. 8, laser beams are distributed not only in the gain region in the active layer 44, but also in the saturable absorbing region outside the gain region. For the purpose of causing self-pulsation, an overlapped area of the laser beam and the saturable absorbing region is required to be as wide as possible. In order to obtain a wide overlapped portion, it is necessary to enlarge the saturable absorbing region by making the active layer 44 thick, or to expand a region in which the laser beam is distributed in a lateral direction (hereinafter, referred to as the "laser beam distributed region") by making the non-ridge portion of the second cladding layer 45 thick. However, if the overlapped portion of the laser beam distributed region and the saturable absorbing region becomes wide, the following two problems arise.
First, when the active layer 44 is thicker, a light confinement effect in a vertical direction becomes strong. As a result, a radiation angle .theta..perp. of laser beams in a vertical direction becomes undesirably wide.
Generally, a radiation angle .theta..parallel. of laser beams in a horizontal direction is typically about 8.degree. to about 10.degree., and this is determined by the width of the striped ridge. On the other hand, the radiation angle .theta..perp. of laser beams in a vertical direction depends on the thickness of the active layer 44, and can be about 40.degree. in the case where the active layer 44 has a thickness necessary for achieving self-pulsation.
As the radiation angle .theta..perp. of laser beams in a vertical direction becomes large, the ellipticity of radiated laser beams becomes large. Excessively large ellipticity causes the deterioration in the efficiency of a lens conversion, thus presenting a disadvantage for use as a light source for an optical disk. On the other hand, when the active layer 44 is made thinner for the purpose of reducing the ellipticity, self-pulsation does not occur.
For example, according to the results of an experiment by the present inventors, an active layer having a multiple quantum well (MQW) structure including a well layer having a thickness of about 6 nm requires 8 or more well layers for causing self-pulsation. Self-pulsation does not occur with 7 or less well layers. On the other hand, in order to obtain a preferable ellipticity as light source for an optical disk, it is preferable that the number of well layers is 7 or less.
Secondly, there is a problem regarding a doping level of the second cladding layer 45.
Generally, it is known that the overflow of carriers from the active layer 44 can be suppressed by increasing the doping level of impurities in the second cladding layer 45. The overflow of carriers occurs more easily as the operating temperature of the semiconductor laser device becomes higher. This is because the kinetic energy of the carriers increases at a high temperature. When the carriers overflow from the active layer 44, invalid currents increase, resulting in an increase of operating current. When the doping level of impurities in the second cladding layer 45 increases, the barrier height of the second cladding layer 45 to the active layer 44 can increase. Thus, the increase of the doping level of the impurities in the second cladding layer 45 is effective for preventing the overflow of the carriers.
However, in the semiconductor laser device having the structure shown in FIG. 8, when the doping level of the second cladding layer 45 increases, an electrical resistivity of the second cladding layer 45 lowers. Thus, the current injected from the electrode flows through the non-ridge portion of the second cladding layer 45 in a spreading manner in a lateral direction. For example, when the second cladding layer 45 is doped with p-type impurities of about 1.times.10.sup.18 cm.sup.-3 or more, self-pulsation cannot be achieved. This is because current flows into the saturable absorbing region by the spread of the current in the lateral direction, and a gain is generated therein, resulting in the disappearance of the light absorbing function. As a result, the saturable absorbing region no longer functions as such. More specifically, the region which can function as the saturable absorbing region moves to the direction further apart from the laser beam distributed region, and interaction between the laser beams and the saturable absorbing region lowers significantly.
Accordingly, for the semiconductor laser device having the conventional structure as shown in FIG. 8, the doping level of impurities in the second cladding layer 45 cannot be increased. For this reason, it is difficult to perform an operation at a high temperature due to the overflow of carriers.